In laser systems it is often desirable to use or provide light at a particular wavelength. By controlling or establishing the wavelength of the laser light, the mode of operation (e.g., Q-switched) can be optimized and the optical noise in the output can be reduced (e.g., reduction in competing modes). In addition, in those systems wherein laser light is transmitted through an optical fiber, the laser's output is preferably at the zero dispersion wavelength or point of the optical fiber in that system.
Diode-pumped neodymium lasers have been shown to be very useful as high powered laser sources for fiberoptic video and microwave transmission. The laser is typically used as a stable continuous-wave (CW) source for an external modulation device, such as a lithium niobate waveguide modulator. In this application it is important that there be no extraneous noise introduced by the laser. The laser must operate in a single transverse mode to eliminate any transverse mode beating. Similarly, the laser must be polarized to avoid beating between frequency-shifted orthogonal modes. Polarization control can also be important for other reasons since, for example, external modulators are often polarization-sensitive. It is also necessary to eliminate mode beating between adjacent longitudinal modes. This can be accomplished by reducing the length of the laser cavity. For example, a 30 GHz bandwidth can be achieved by reducing the round trip cavity optical path length to less than 1 cm. However, cavities this short can be difficult to build, especially when components such as Brewster plate polarizers are used; these components tend to increase the cavity length considerably. Minimization of the laser cavity length is usually easiest in the case of a monolithic laser cavity.
If the laser system works correctly, then the bandwidth of the fiberoptic system is determined by the fiber dispersion properties. The propagation velocity of an optical pulse in a fiber is determined by the group velocity "v" which is defined by v=c[d(n/.lambda.)/d(1/.lambda.)]. In this expression, "c" is the speed of light, ".lambda." is the wavelength and "n" is the effective refractive index for the fiber. Because of waveguide effects, the effective value of "n" is dependent on the fiber geometry and it may not be exactly equal to the value for bulk fused silica. The wavelength dependence of "v" is usually described in terms of the dispersion "D" which is defined as D=dv/d.lambda.. Dispersion is usually expressed for fused silica fibers by an empirical formula, such as D(.lambda.)=(S.sub.0 /4)[.lambda.-.lambda..sub.0.sup.4 /.lambda..sup.3)] wherein values for S.sub.0 and .lambda..sub.0 are supplied by the fiber manufacturer. When .lambda.=.lambda..sub.0, D(.lambda.) equals zero; this is the "zero dispersion wavelength" of the fiber and is typically between 1300 nm and 1320 nm.
In many cases, the optical pulses carrying information through the fiber are not monochromatic; they have a spectral width ".DELTA..lambda.". This width is typically defined as the "Full Width at Half Maximum" (FWHM) value, although with a multi-longitudinal mode laser source, the actual wavelength distribution usually consists of several discrete peaks. Because of dispersion, the different wavelength components of the pulse will propagate with different velocities. This will cause a pulse to broaden as it travels through the fiber. The resulting distortion of the optical signal will reduce the signal modulation depth without reducing the total power. This distortion is characterized by the pulse spread, ".DELTA.t" which is defined by .DELTA.t=D(.lambda.)[.DELTA..lambda./L], where "L" is the optical fiber length. It represents the difference in transit times between two pulses with wavelengths .lambda.-(.DELTA..lambda./2) and .lambda.+(.DELTA..lambda./2). When .lambda.=.lambda..sub.0 (i.e., the laser pumps the optical fiber at the zero dispersion wavelength), D(.lambda.)=0 and .DELTA.t=0.
The information bandwidth for the fiber is determined primarily by the pulse spread. If an optical source, with a wavelength distribution consisting of equal peaks at .lambda.-(.DELTA..lambda./2) and .lambda.+(.DELTA..lambda./2), is amplitude modulated with a sine wave at a frequency f=1/(2.DELTA..tau.), the modulation depth of the output will be zero. This occurs because the output at .lambda.-(.DELTA..lambda./2) is phase shifted by half a period with respect to that at .lambda.+(.DELTA..lambda./2). The loss of modulation is less dramatic if a more uniform wavelength distribution is assumed; it is only a factor of two, if a spectrally uniform pulse is used. As a result, the frequency f=1/(2.DELTA..tau.) is generally referred to as "the 3-dB optical bandwidth" of the fiber. The fiber transmission bandwidth can be maximized by minimizing either D(.lambda.) or .DELTA..lambda.. Preferably, a laser source is selected for supplying the optical fiber such that the laser operates as close as possible to the zero dispersion wavelength (e.g., 1301.5 to 1321.5 nm) of the optical fiber or at D(.lambda..sub.0)=0.
Another problem which occurs in fiber optic applications is that of Stimulated Brillouin Scattering (SBS). Aoki et. al., J. Opt. Soc. Am. B., 5(2), 358-363, (1988). SBS is a nonlinear loss mechanism which becomes important when high power laser sources are propagated through long, low loss optical fibers. The spectral output of a diode-pumped neodymium laser typically consists of several modes with kilohertz linewidths spaced at intervals of several gigahertz. As shown by Aoki et. al., under these conditions, SBS becomes a problem when the power-per-mode exceeds a certain threshold value. Thus, the maximum allowable power in the fiber is determined by the number of oscillating modes in the pump laser as well as the manner in which the power is distributed between these modes. A laser which satisfies these requirements can be accomplished by using a laser material with a relatively broad gain peak, so that several longitudinal modes can operate despite the large frequency spacing required. Of course, the lasing spectrum of a laser can be much narrower than the width of the gain peak. The effects in the laser which tend to drive the laser into multi-longitudinal mode operation (e.g., spatial hole burning) must be carefully optimized if the operating linewidth is to be maximized.
Thus, there are at least three factors which determine the optimum spectral configuration of a laser for fiberoptic applications in the GHz region. Mode beating noise requirements lead to the condition that the mode spacing be as large as possible. The conditions required to suppress SBS suggest the use of a laser material with a relatively broad gain peak, so that several widely spaced longitudinal modes can operate. Finally, the requirement of a large fiber transmission bandwidth leads to the condition that D(.lambda.).DELTA..lambda. be minimized. In the presence of the required large value of .DELTA..lambda., this can be satisfied only if the laser is operated at a wavelength where D(.lambda.) is zero. Thus, there is a need for a polarized laser having a short laser cavity which can be made to operate on several widely spaced longitudinal modes and at the zero-dispersion point of the fiber.
Diode-pumped neodymium lasers are very useful high-power sources for fiberoptic video transmission. One particularly useful source of laser light is laser diode pumped, neodymium-doped yttrium lithium fluoride (Nd:YLF or NYLF). The strong 1 .mu.m transitions at 1047 nm and 1053 nm are widely used for applications such as Q-switching, mode locking, and intracavity doubling. The weaker 1.3 .mu.m transitions at 1321 nm and 1313 nm are also of interest; the 1313 nm line is particularly interesting for fiberoptic applications since it is very close to the zero dispersion wavelength .lambda..sub.o in silica fibers.
Birefringent laser crystals, such as Nd:YLF, are characterized by an optical ellipsoid and typically have strongly polarization-dependent gain and absorption spectra. In Nd:YLF, the strongest 1 .mu.m (.sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.11/2) transitions are at 1047 nm (.sigma..sub..pi. =18.times.10.sup.-20 cm.sup.2), and 1053 nm (.sigma..sub..sigma. =12.times.10.sup.-20 cm.sup.2). The corresponding 1.3 .mu.m (.sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.13/2) transitions are at 1313 nm (.sigma..sub..sigma. =3.times.10.sup.-20 cm.sup.2) and 1321 nm (.sigma..sub..pi. =3.times.10.sup.-20 cm.sup.2). In addition, the 800 nm absorption spectrum is strongly polarized. In 1% Nd:YLF the absorption spectra consists of two main peaks, at 792 and 797 nm, with absorption coefficients of .alpha..sub..sigma. =1 cm.sup.-1, .alpha..sub..pi. =9 cm.sup.-1 at 792 nm and .alpha..sub..sigma. =3 cm.sup.-1, .alpha..sub..sigma. =6 cm.sup.-1 at 797 nm. This makes Nd:YLF easy to pump with a laser diode emitting light at about 800 nm.
Other lasers use a host crystal of yttrium aluminium perovskite (YAIO.sub.3 or YAP or YALO). The polarization dependent gain in YALO has been reported, and the lasing characteristics for pumping along principal crystal axes, through the addition of a polarization selective loss in the cavity (e.g., by employing a polarization and wavelength selector, such as a Brewster prism) has been investigated. G. A. Massey et al, Appl. Phys. Lett., Vol. 18, No. 1 (1971). G. A. Massey, Jour. Quantum Electron., Vol. QE-8, No. 7 (1972), p. 669-674, and A. Abramovici, Optics Comm., Vol. 61, No. 6 (1987), p. 401-404. Others have reported the variation of the florescence spectra with polarization and wavelength selectivity in laser cavities employing YALO. M. J. Weber, et al., Appl. Phys. Letts., Vol. 15, No. 10 (1969), p. 342-345. It has been suggested that the anisotropy of the stimulated emission cross sections of the principal axes, for various transitions of Nd doped YAP, can be used in tailoring Nd:YAP for specific laser applications. M. J. Weber, Appl. Phys., Vol. 42, No. 42, (1971), p. 4996-5005. In particular, improved Q-switched operation in a Nd doped YALO laser has been studied, and the dependence of the gain coefficients for Nd in YALO upon the crystallographic orientation of the lasant rod's principal axes and the selection of the crystal axis with optimum gain characteristics has been reported. M. Bass, et al., Appl. Phys. Letts., Vol. 17, No. 9 (1970), p. 395-398. Similar studies of YLF have not been found.
Nd:YLF lasers are typically operated with a ninety degree angle ".theta." between the optic axis (i.e., crystal c-axis) and the propagation axis; propagation is along the a-axis, pumping is into the .pi. polarized absorption peak at 792 nm and lasing is with the .pi. polarized 1047 nm transition. With 1 .mu.m cavity reflectors, this usually leads to .pi.-polarized 1047 nm emission with powers between 50 and 100 mW. The output is typically linearly polarized with a polarization ratio of more than 1000:1. Reliable single-line operation at either 1047 nm or 1053 nm can be achieved by aligning a Brewster plate to the .pi. or .sigma. axes of the crystal. Comparable powers can be obtained at either wavelength. Unfortunately, the output polarization is quite sensitive to the alignment of the axes of the Brewster plate to the crystal axes. As can be shown from a Jones matrix analysis, any misalignment results in an elliptically polarized output. Moreover, not only does the Brewster plate increase the overall size/length of the laser source, but also it can decrease the useful power.
1053 nm lasing, .sigma.-polarized, of Nd:YLF can be achieved with .theta.=0 degrees (i.e., propagation along the c-axis); however, this has two problems. First of all, the laser is no longer polarized (i.e., it operates in both available polarizations). For mechanical reasons, it is almost impossible to ensure that propagation will be exactly along the c-axis; this leads to a slight degree of birefringence which splits the laser into two sets of orthogonally polarized modes, both of which tend to lase. A second problem relates to the fact that if the laser is propagating along the c-axis, the output power is typically 20% lower than the laser operated with a ninety degree angle to the optic axis. (i.e., .theta.=90.degree.). This appears to be due to inefficient pumping, since with .theta.=0, only the relatively weak .sigma.-polarized spectrum is accessible. In this configuration, it is also difficult to control the laser's polarization. Adding a Brewster plate to the cavity can suppress one of the polarizations; however, it must be aligned exactly with the difficult-to-find-axes of the crystal, otherwise it will become a Lyot filter. (e.g., Ambramovici, supra, for Nd:YAP) In general, it is easier to add the Brewster plate to the a-axis rod, which is easy to find.
Similar polarized Nd:YLF laser systems can be constructed to operate at 1313/1321 nm. With 200 mW input power, outputs between 25 and 50 mW are typically achieved. When a Nd:YLF laser is made to operate with .theta.=90 degrees without a Brewster plate, the laser operates simultaneously on both the .sigma.-polarized 1313 nm line and the .pi.-polarized 1321 nm line. The two lines have virtually identical gain, and they both tend to lase.
This two-line operation is typical of many laser materials at 1.3 .mu.m. For example, a Nd:YAG laser operating at 1319/1338 nm and a Nd:GGG laser operating at 1323/1331 nm. In many materials, single line operation can only be achieved if an additional wavelength-selective element such as a Lyot filter or an etalon is added to the laser cavity. In the case of Nd:YLF, polarization control with a Brewster plate can be used to achieve single line operation at either wavelength. 1313 nm operation can also be achieved by propagating along the c-axis (i.e., .theta.=90.degree.), with all the same pumping and polarization problems as the c-axis propagating 1053 nm laser system.
U.S. Pat. No. 3,624,545, issued to Ross on Nov. 30, 1971, describes an optically-pumped solid state laser composed of a Nd:YAG rod which is side-pumped by at least one semiconductor diode laser. Similarly, U.S. Pat. No. 3,753,145, issued to Chesler on Aug. 14, 1973, discloses the use of one or more light-emitting semiconductor diodes to end-pump a neodymium-doped YAG rod. The use of an array of pulsed diode lasers to end-pump a solid lasant material, such as neodymium-doped YAG, is described in U.S. Pat. No. 3,982,201, which was issued to Rosenkranttz et al. on Sept. 21, 1976. Finally D. L. Sipes, Appl. Phys. Lett. Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tightly focused semiconductor diode laser array to end-pump a neodymium-doped YAG results in a high efficiency conversion of pumping radiation having a wavelength of about 810 nm to output radiation having a wavelength of 1064 nm.
Thus, although the art has recognized the polarization-dependent gain and absorption spectra of some birefringent laser crystals, such crystals are typically operated at .theta.=0 or .theta.=90 degrees. More importantly, the efficiencies to be gained by operating an optical fiber at the zero dispersion wavelength .lambda..sub.o has not been linked, in general, to the orientation of a laser crystal relative to the axis of propagation or to how that orientation can be used to optimize the performance of the laser's mode of operation, reduce its optical noise or tailor its optical spectrum.